Dear Audio Processing Forum People,

sometimes we need to sample audio signal.

Maybe some old favorite vinyl LP?

The Shanon-Nyquist sampling theorem says,

that in order to get information about frequency f,

you must use sampling rate at least 2f.

Puting this backward, the upper half of your Fourier amplitudes

are false (in fact they are complex conjugates of the lower half).

So we think like this: the human built-in microphone,

ie, the human ear, recognizes frequencies from 20Hz to 20KHz,

therefore, the sampling rate 40Khz (or 44,1Kz) is just ok.

Similar thinking applies to voice recording.

Voice range is up to 4KHz, so rate 8KHz seems good.

But the actual experiments show the we can obtain better

quality if we sample the signal with much higher rate,

compute Fourier, and INVERSE Fourier, AND re-sample

with lower (final) rate.

For example, first sample voice audio by 20KHz (say),

compute FFT, the inverse FT and re-sample this inverse using 8KHz.

My question is, what is the reasonable and best choice

for this higher frequency?

It would be stupid and unpractical to sample voice using 1Mhz!

I think, that this is a fundamental practical question in

good quality sampling.

But I believe that this Forum is the best one in the world…

j.